The present invention relates in general to signal evaluating equipment and in particular to a new and useful arrangement for evaluating an optical beam which has passed through a Bragg-cell, in particular a Bragg-cell which has been exposed to radio frequency signals.
An arrangement to evaluate optical beams is known from the article "Bragg-Cell RF-Signal Processing" by Coppock et al, Microwave Journal, September 1978, pages 62-65. In such an arrangement, an RF signal to be analyzed produces sound waves in a Bragg-cell causing the deflection of a light beam passing through the Bragg-cell. Advantageously, a coherent light beam, for example a laser beam, is used. The diffraction patterns produced by the deflected light beam are to be considered as Fourier transforms of the RF signal into the frequency range (frequency range here corresponds to angular range) and are usually evaluated by means of photodetectors, to measure the spectral power density of the RF signal.
With Bragg-cells of high resolution, several hundred points must be evaluated. For this purpose, so-called "photodiode arrays" are usually employed which have a serial display (digital or analog) through connected shift registers. The signals of the photodetectors are available in time sequence (in phase with the shifting cycle of the shift register), i.e. always only a single signal of a photodetector (corresponding to a frequency window of the RF signal) is available. The intensity of the signal (photocurrent of one photodetector) corresponds to the integral ##EQU1## wherein T=integration interval of the photodetector, and P.sub.RF (t) a variation in time of the signal amplitude during the integration interval. The integration interval T of the photodetector may be taken as being about N..tau. with N being the total number of photodetectors, and .tau. being the duration of the shifting cycle at the sequential readout. Thus, not the true spectral power of the RF signal, but the integral power within a frequency window is determined corresponding to the angular range covered by a single photodiode. If, as in an RF monitoring reception for example, a plurality of pulse modulated transmitters is concerned, having overlapping spectra and/or pulse repetition rates smaller than the integration interval T, neither the power nor the spectrum of the individual transmitters can be inferred from the photocurrent of a single photodiode associated with a definite frequency window.
Another disadvantage is that the limiting sensitivity (minimum detectable RF power or brightness in the angular interval) of photodiode arrays is poor. This is primarily due to the small photoactive areas of the photodiode arrays.